Happy days: A module is perfect for young plants to grow in. Photograph: Alamy

Last week I wrote about how a little warmth from a propagator can allow you to grow all sorts of plants on your windowsill. Once those seedlings are up, you need to move swiftly on to the next stage to raise healthy plants. If you leave your seedlings in cramped conditions, you're inviting trouble.

Air circulation is important for young seedlings. Although they need moist conditions to grow, if they're too crowded it's the perfect environment for "damp off". Damp off is a single term used to describe a number of soil-borne diseases. The symptoms can be quite dramatic. Seedlings will suddenly wilt, the bottom of the stem will shrivel, collapse or die, turn yellow, weaken and generally give up the ghost. This happens below soil level, so all you'll see is poor or uneven stands of seedlings.

Overcrowding and overwatering are the biggest culprits, but dirty pots and trays or unsterilised soil can also be an issue.

Prick out seedlings as soon as they're large enough to handle. Handle seedlings by their baby leaves, never the stem which is easily damaged. Seedlings should be placed 2-7cm apart, depending on the plant. I prefer to use modules as this gives each plant equal space. Young seedling roots that have become overcrowded often grow into each other and when you separate them, the delicate root hairs can be damaged. A module is perfect for the young plant to grow in and makes the transition into the ground easier.

To increase air circulation, remove propagation covers on sunny days to stop too much condensation. If possible, water from below as this also helps keep the soil surface drier.

Wash all your pots and trays. I have to admit I use my dishwasher. If this horrifies you, never come to tea at my house, and wash them with Citrox or similar natural disinfectant. Most store-bought seed compost is sterilised, but it is important to sterilise homemade potting compost. Sieve the soil, place it in a suitable container and pop it in the microwave for a couple of minutes on high power or in the oven at 80C for an hour.

Camomile tea is said to be a natural fungicide to damp off. Let a camomile tea bag steep until completely cool and water with this once a day to keep funguses at bay.

But the real trick is to keep air circulating around the plant. You don't need to aim a hairdryer at them, but a gentle brush with your hand, turning the tray around and a little movement will do wonders for their health.

Alys on autumn-fruiting raspberries

Photograph: Getty

There are two types of raspberries: summer- and autumn-fruiting. If you ate raspberries in June, you have a summer-fruiting variety. If you started eating raspberries in August and continued to eat them in October, you have an autumn-fruiting variety. There is only one widely available autumn cultivar, so you've most likely got 'Autumn Bliss'.

This type of raspberry is pruned now (summer ones are pruned in summer after fruiting, so we'll wait until then to talk about those). Autumn-fruiting raspberries fruit on this year's growth and are very easy to look after. All you need to do is prune the canes down to ground level. Do this before any growth occurs. Once the new canes appear, remove any that are weak or damaged. Now all you have to do is sit back and wait until late summer to harvest.

Unless you're in a very exposed area, I'd argue against tying in the canes. Left to their own will, the plants will bend over as if they are bowing, and thus the fruit will be hidden from birds. Make them stand up straight and you'll expose the fruit to all and sundry.